Allow user to specify a version, and update all files for it

[fingolfin]

So say you do ./release-gap-package 1.2.3

Then it could update PackageInfo.g and set that version in there, and the current date, and commit that. It'd possible also update CHANGES.md (see also issue #19, and my limited implementation in https://github.com/oscar-system/GAP.jl/blob/master/etc/update_version.sh)

Of course ideally it should also verify the package version actually increases.